Three Oakland start-up companies are among semifinalists in the national Clean Tech Open that began Thursday in Mountain View.

That same day, Oakland-based Lucid Design Group was named by a panel at the Rio+20 United Nations Conference on Sustainable Development as among 100 companies worldwide creating sustainable solutions, dubbed Sustania 100. Lucid makes software that allows buildings to reduce energy use. 

Start-ups Zeo Mundas, Vegetated Cool Roof and Smart Energy were selected at the Clean Tech Open as semifinalists along with Berkeley firms Strider Solar and Malpas Sustainable Design and 42 other start-ups from around the nation as worthy to participate in a business accelerator program to help them move from idea to business. From Friday to Sunday, they will participate in intense business training, including help with fine tuning a pitch to bring to investors, and have a shot at competing for monetary prizes as finalists. 

"We've got a really good idea of where we want to take this but it is going to take some capital," said Amelia Holmes, co-founder of Vegetated Cool Roof.  Their business is about building green roofs in a unique way that does not involve putting soil on rooftops. Prototypes of the Vegetated Cool Roof have shown energy use reduction of 18 percent for houses in hot dry climates of cities like Fresno. 

Oakland firms Smart Energy and Zeo Mundus both competed in the Air, Water, and Waste category. Officials from neither firm, however, were available at the Clean Tech exhibit hall. Founder of Strider solar, Za'ev Abrams, said his firm's product captures sunlight landing between solar panels and refracts it towards the panels.   

That Oakland was represented among the start-ups, which predictably had a high concentration of Silicon Valley addresses as well as Seattle and Austin, Texas, was testament to the city's nascent green industry sector. Abrams of Strider Solar said that when an area holds a critical mass of like companies it has the services needed to support those companies. Still, the Oakland-Berkeley area may not yet have enough of a clean tech critical mass, since Strider Solar will soon be moving to Mountain View, he said.

The wide range of innovations at the Clean Tech Open exhibit hall included a firm using recycled plastic bags and milk cartons mixed with limestone to make home construction material including roof shingles and outdoor siding. Another firm offered material for a build-it-yourself electric car and another is making small wind turbines fixture for trains to capture the wind tunnels created at their sides when they pull into stations.

The chance to compete in the Clean Tech Open can give a firm a leg up. For instance Lucid Design Group was the 2007 winner of the Clean Tech Open in the smart power category.